Here are some of URLs by using them you can test your website HTML/XHTML compliance, CSS compliance, Speed test and website performance.
(click on the image to open the website)
Wanted to know different screen sizes for different devices (phone, tablets, monitors) here is the complete listing for you.
Wanted to check the website color scheme? Most common color or hue on the web? It is not that difficult now just simply enter the website url and get to know all the details with single click on get colors button. It will give you the color breakdown, hue distribution, color wheel and websites with similar colors.